In less than a decade, the state of the art in fiber-optic transport systems has progressed from simple point-to-point chains of optically amplified fiber spans to massive networks with hundreds of optically amplified spans connecting transparent add-drop nodes spread over transcontinental distances. Cost reduction has been the primary driver for this transformation, and the primary enabler has been the emergence of the ROADM as a network element (NE).
Exploiting the inherent wavelength granularity of wavelength-division multiplexing, an optical add/drop multiplexer (OADM) allows some WDM channels (also referred to as wavelengths) to be dropped at a node, while the others traverse the same node without electronic regeneration. Previously, it was necessary to terminate line systems at each node served, and then regenerate the wavelength signals destined for other nodes. The ability to optically add/drop a fraction of a system's wavelengths at a node was first achieved using fixed OADMs. These were constructed from optical filters, and by enabling wavelengths to optically bypass nodes and eliminate unnecessary regeneration, they provided significant cost savings. However, because traffic growth is inherently unpredictable, it is advantageous for the add-drop capability to be reconfigurable.
ROADMs provide many advantages beyond the savings achieved by optically bypassing nodes. In the future, multi-degree ROADMs with adequate reconfiguration speeds may enable shared-mesh restoration at the optical layer. Shared mesh restoration significantly reduces the number of wavelength channels that must be installed as redundant protection circuits. ROADMs also provide operational advantages. Because ROADMs can be reconfigured remotely, they enable new wavelength channels to be installed by simply placing transponders at the end points, without needing to visit multiple intermediate sites. In addition to these cost-saving benefits, ROADMs will enable new services. For example, if transponders are preinstalled, then new circuits can be provided on-demand. The rapid network reconfiguration provided by ROADMs could also become an enabler of dynamic network services, such as switched video for IPTV. For all of these reasons, ROADMs will continue to have a significant effect on the design of optical networks.
FIG. 1 is a schematic illustration of a prior art ROADM node 100 showing the control loops employed in a typical configuration. The drawing depicts one direction of a typical multi-degree ROADM node, which includes multiple control loops to control the power of wavelengths through the node. In today's networks, optical links are typically bidirectional, so each port really represents a pair of fibers. When using conventional local transceivers that can process only a single wavelength at a time, the number of fibers in the add/drop port sets the maximum number of wavelengths that can be added or dropped at a given node. Wavelengths from a remote ROADM node enter on the network fiber port 102 on the left prior to dropping an Optical Supervisory Channel (OSC) at 104. The signals enter one optical amplifier 106 or a series of optical amplifiers 106, 110, which may be preceded by a variable attenuator 108 to control the input power. Multiple optical taps with photodetectors (PD), each shown as 112, are used to monitor the power and support active control of the amplifier input power 114 and gain 116. The wavelengths are then split using a passive optical splitter (PS) or wavelength selective switch (WSS) 118 to either drop at the demultiplexer 120, or connect to an output network degree via couplings shown at 122. Wavelengths added to the ROADM 100 may either be provided by external wavelength sources or local transponder(s) 124, which may include laser power control loop 126 and wavelength control loop 128. A WSS 130 for each network output is used to combine the add wavelengths from a multiplexer 132 and wavelengths from the input network degrees 134. WSS 130 also provides per-channel variable attenuation. An optical channel monitor 136 at the output of the WSS measures the power of each wavelength, and this data is used to adjust the attenuation of the WSS to balance the channel powers. The wavelengths then typically pass through an output optical amplifier 138, which may also include gain control loop 140 or output channel power control loop 142. Finally, an OSC channel 144 may be added to the outgoing optical signal as it exits the ROADM node 100. A similar optical path and control loops are also typically used by 2-degree ROADM nodes (not shown), where the multi-degree splitter and WSS are replaced by 2×1 components that also provide per-channel variable attenuation (e.g., a PLC ROADM). A control loop processor (CLP) 146, shown as a single element but alternatively configured in multiple specific individual elements, executes control loop functionality based in part on inputs 148 (represented by down arrows) and the channel power control loop (CPCL) configuration, for example computer code, which results in outputs 150 used to adjust or modify specific ROADM Node 100 elements and/or overall performance or functionality. Examples of inputs to the CLP 146 are information or data from elements such as PDs 112 and OCM 136. Examples of outputs (represented by up arrows) from the CLP 146 are information or data to control elements such as optical amplifiers 106 and 138, and variable attenuator 108.
As shown in FIG. 1, multiple control loops are used along the wavelength path through a ROADM node to compensate for span loss changes, adjust amplifier gain, control transponder output power and wavelength, and balance channel power levels to compensate for wavelength dependant losses and amplifier gain tilt. A wavelength channel within a ROADM network can therefore traverse a large number of cascaded control loops as it passes through multiple nodes. While the ROADM system should be designed to avoid unwanted interactions between these cascaded control loops and prevent cross-channel interference in the event of a failure within the network, this has proven to not always be the case. The response times, power thresholds, etc. for these control loops are typically designed based on the system features and requirements envisioned at the time of the initial design and for known failure mechanisms.
However, new or modified features and requirements as well as unforeseen failure mechanisms have shown the need for a capability to both detect unstable wavelengths and protect against cross-channel interference. This will continue to become more important as ROADM systems continue to increase in tenability and flexibility and support alien wavelengths.
One specific example is a case where an unexpected failure mode within a tunable laser results in very rapid transitions between two lasing modes (wavelengths). This typically results in oscillations between the states within the channel power control loop and very rapid and large fluctuations in the channel power. This then impacts other channels at the downstream amplifiers. Troubleshooting this type of failure is complicated by the impact on multiple channels that add and drop at different nodes within the ROADM network, making it difficult to isolate the failed channel.
Current CPCL configuration of ROADM networks typically responds to the out-of-range instantaneous channel input power level by simply blocking and opening/attenuating of the channel. This may or may not actually solve the problem with the channel, but may instead be a symptom of the problem. For example, if there is a problem with an amplifier which is amplifying the signal prior to being received at a ROADM, the channel input power level could be excessively high, and the receiving ROADM with existing CPCL configuration would simply block the channel, and reopen it at a later time. In this example, blocking the channel does not solve the problem of the troubled amplifier or give the network administrator/engineer an indication that there even is a problem. Additionally, network capacity is reduced, as the fiber (or fiber pair, as determined by network configuration) is now no longer carrying network traffic.
FIG. 2 shows a typical prior art state diagram 200 for the power control loop of each output wavelength channel that monitors the channel power and controls the variable attenuation of the output WSS in FIG. 1. The system starts at the Shutdown state 202, powering up, initializing the Out-of-Service/Blocked state 204 with the maximum channel attenuation. When a wavelength channel between an input and output port is provisioned in-service, the corresponding WSS connection is established in the High Attenuation state 206 where the presence of the channel can be detected without risk of impacting other wavelengths on the network. When the channel optical power exceeds the channel detect threshold level, the control loop enters the Channel Add state 208, where the attenuation is slowly decreased until the target optical power level is achieved. Once the target power level is reached, the state transitions to the Channel Level Control state 210 where the attenuation is adjusted to maintain the power within the operating window. The channel may be provisioned out of service by going directly from the Channel Level Control state 210 to the Out-of-Service/Blocked state 204, where the system can then be either powered down by going to the Shutdown state 202 or a channel can be re-provisioned in service per the High Attenuation state 204. Alternatively, while at the Channel Level Control state 210, if the power moves outside this window (e.g., loss of light) the control loop will return to the High Attenuation state 208 to either provision the channel out of service, going to the Out-of-Service/Blocked state 204 and then to the Shutdown state 202, or try and detect channel power, moving to the Channel Add state 208 with the goal of reaching target power and returning to the Channel level Control state 210. Thus, the system can repeatedly go through the cycles of blocking, opening and attenuating the input power depending on the instantaneous power detected.
For managing the performance of high capacity optical switching and transmission WDM networks, optical performance monitoring (OPM) systems, sometimes referred to as an optical channel monitor (OCM) are used. OPM involves determining the quality of optical channel(s) within the WDM network by measuring optical characteristics without examining the transmitted sequence of data bits, assuring data security. OPM may include ensuring correct switching in ROADMs, setting levels for dynamic equalization of the gain of optical amplifiers and providing system alarms and error warning for lost or out of specification optical channels. Typical parameters measured are channel power, polarization dependent loss, wavelength and optical signal-to-noise ratio (OSNR) for each channel.
FIG. 3 is a high-level flow diagram 300 of a prior art channel power control loop. This diagram is stated in terms of a MEMs based WSS (i.e., mirrors used for channel connections and attenuation control), but would also apply for other WSS technologies, as known by those skilled in the art. Steps 302 and 304 comprise the Out-of Service/Blocked state. The flow method 300 starts when the system is powered up and the mirror is moved to the blocked channel position 302. In step 304, a channel provisioned in-service check is performed before moving to step 306.
Steps 306 through 310 comprise the High Attenuation state. At step 306, the mirror is moved to the High Attenuation position for provisioned input/output port pairing. At Step 308 the channel power is measured. At step 310, if the channel power is not above the channel-detect threshold, the process returns to step 308 where the channel power is measured. If at step 310 the channel power is above the channel-detect threshold, the process moves to step 312.
Steps 312 and 314 comprise the Channel Add state. At step 312 the channel attenuation is decreased by one incremental step. At step 314, if the target channel power (Ptarget) is not reached, the process returns to step 312 where the channel attenuation is again decreased by one incremental step. If at step 314 the target channel power (Ptarget) is reached, the process moves to step 316.
Steps 316 through 326 comprise the Channel Level Control state. At step 316 the channel power (Pmeas) is measured. At step 318, if Pmeas is not between Max and Min power thresholds, the process moves to step 320 where the threshold crossing alert is raised, and the process returns to step 306. If at step 318 Pmeas is between Max and Min power thresholds, the process moves to step 322. At step 322, if (Pmeas−Ptarget) is greater than attenuation step increment, the process moves to step 324 where the attenuation is adjusted to set the channel power to the target level, and the process moves to step 324. If at step 322 (Pmeas−Ptarget) is not greater than attenuation step increment, the process moves directly to step 326. At step 326, if the channel is not provisioned Out-of-Service, the process returns to step 316. If at step 326 the channel is provisioned Out-of-Service, the process returns to step 302 where the mirrors are moved to the blocked position.
In other words, once the system is in the Channel Power Control state, if the power level measured by the optical channel monitor crosses the Max or Min operating level threshold the WSS will return to the High Attenuation state. The channel will automatically recover if the power level moves back above the channel detect threshold. Note: Channel Detect Threshold (dBm)>(Minimum Operating Threshold (dBm)−Attenuation Added for High Attenuation State (dB)+Maximum Power Measurement Error (dB)).
An unstable wavelength channel on a ROADM network can create power level surges which may not only impact the channel directly, but may impact other channels on the same network as well. Current CPCL configurations in a ROADM network, within a device such as a CLP 146 of FIG. 1, are designed and implemented to block and open a wavelength channel based primarily on the instantaneous channel input power level. Certain failure modes on a wavelength channel can cause the CPCL to repeatedly go through the blocking and opening cycles and/or repeatedly create excessive power fluctuations (sudden surges and falls) on the channel output to the extent that performance parameters, those discussed earlier, of other channels on the ROADM network may be impacted.
Excessive optical power fluctuations, i.e., surges over X db, where X depends on the CPCL configuration of the CLP for the specific ROADM system, on some wavelength channel(s) can interfere with other channels on a ROADM network to the extent of network-wide degradation and outages. Current CPCL configurations typically react in an uncorrelated manner to input power changes that occur within a specified time period, i.e. milliseconds to seconds. If the input power fluctuations occur repetitively at a certain frequency, either in or out of the control loop cycle range, the typical CPCL configuration cannot recognize the pattern and will simply go through repeated cycles of blocking, opening and attenuating of the input optical power depending on the instantaneous power detected. Network outages have been reported in the field with lengthy manual trouble isolation and service restoration efforts involved as a result of certain laser failure mode that repeatedly created input power surges into the control logic.
It would therefore be desirable to provide systems and methods that improve the CPCL configuration of a ROADM network, with the ability to detect an unstable wavelength channel by comparing actual performance patterns to pre-determined performance patterns and rules to recognize and respond to the patterns and rules, and by taking pre-determined action(s) to avoid network-wide degradation and outages. To the inventors' knowledge, no such system or method currently exists.